The background image is shown in the browser, and extends for the size of the page.
The size of the page (which is not the size of the browser window - you could have scrollbars there) depends on how much data needs to be displayed. So it varies. You may want to use a background image that is very high (ex. 3000 pixels) to cover the entire page height without tiling.
Alternatively, consider embedding Bookeo as a widget on your own website. You can then simply use the default background of your website. Click here for instructions.